A person who wears a cock-eyed ball cap with a perfectly flat bill perched askew on one's head.
A fad originally started by black "urban youth" which denoted prosperity (because new hats have unformed bills). Leaving the price tags and labels on was common as well, indicating that the wearer could afford a new hat. This falls under the social phenomena of "nigger rich".
Like most "street fashion", this style has largely been appropriated (or stolen) by privileged white hipsters who want to express solidarity with black urban youth, and indicate that they too are "hard" and "from the streets".
"A brutha can't even flat brim his hat without some white muthafucka always perpetratin' on his style."
"HA HA - that hipster riding his fixie just lost his flat brim when that truck passed him!"